1. Field of the Invention
This invention relates to micropump assemblies for microgas chromatographs and the like.
2. Background Art
In the last decade, a large number of micropump designs have been reported in the literature. Zengerle & Sandmaier provide an overview of early developments of micropumps in 1996 Microfluidics, Proc. Seventh International Symposium on Micro Machine and Human Science, pp. 13–20, IEEE.
Several trends in the design of micropumps are readily identified in the literature. Actuation is a key element of the pump. For gas pumping applications, electrostatically or piezoelectric driven membranes are frequently used. However, these actuation mechanisms are limited by the volume displacement of the membrane and require high drive voltages. The microvalves needed to control the flow in and out of the pump are another critical part of the design. Although valve-less micropumps have been proposed, these pumps have significantly lower performance than micropumps using check valves, particularly for gas operation as described in Gerlach, “Pumping Gases by a Silicon Micropump with Dynamic Passive Valves,” Transducers '97, Proc. International Conference on Solid-State Sensors and Actuators, pp. 357–360 (1997); and Wijngaart et al., “The First Self-Printing and Bi-Directional Valve-less Diffuser Micropump for Both Liquid and Gas,” Proc. 13th Annual International Conference on Micro Electro Mechanical Systems, MEMS 2000, pp. 674–679.
More recently, Cabuz et al. describe an electrostatically actuated dual-diaphragm gas micropump which integrates the microvalves in the moving diaphragm. Typical performance of these pumps, however, would not meet the requirements of many micro gas chromatographs. Cabuz et al., “The Dual Diaphragm Pump,” Proc. 14th IEEE International Conference on Micro Electro Mechanical Systems, MEMS 2001, pp. 519–522. In particular, the maximum flow rate required, which could be as high as 50 ml/min at a pressure rise of a few tens of an atmosphere, cannot be obtained with present designs. However, power consumption of electrostatically actuated pumps is comparatively low of the order of a few milliwatts, which is consistent with the power requirements of microgas chromatographs.
There have been a number of recent developments of electrostatically-driven acoustic jet arrays for micro air vehicle propulsion and control. The requirements for the membranes used in the acoustic jet arrays include a large volume displacement and high operating frequency, as described in Müller et al., “Acoustically Generated Micromachined Jet Arrays for Micropropulsion Applications,” Proc. 2002 ASME International Mechanical Engineering Congress & Exposition, IMECE 2002–33630; and Chou et al., “3D MEMS Fabrication Using Low Temperature Wafer Bonding with Benzocyclobutane,” Transducers, 2001.
The following U.S. patent documents are related to the present application: U.S. 2003/0068231 A1; U.S. Pat. Nos. 6,544,655; 6,328,228; 6,358,021; 6,351,054; 6,288,472; 6,255,758; 6,240,944; 6,215,291; 6,184,607; 6,184,608; 6,179,586; 6,168,395; 6,106,245; 5,901,939; 5,836,750; 5,822,170; 5,529,465; 5,180,288; 5,078,581; and 4,911,616.
Recently, efforts to lower operating power or voltage have attracted attention in most MEMS devices as well as other electronic systems. It is especially true for electrostatically-actuated MEMS devices where the operation is controlled by applied voltage. The maximum out-of-plane (vertical) deflection in flat electrostatic electrode actuators is limited by their small gap separation for an acceptable pull-in voltage. In order to achieve an optimized trade-off between parallel-plate deflection and voltage, a diverse and large number of approaches have been pursued. Among them, the concept of curved electrode by Legtenberg offers several benefits. The main idea is that much larger electrostatic forces, due to smaller air gap at the edges, can be obtained when one electrode of the two parallel electrodes is made to be curved. Thus, the flat membrane can be moved to a much larger vertical deflection with a lower voltage because a large force is created around the edges where the two electrodes are closest. Then, a so-called “zipping” effect proceeds to collapse the membrane against the electrode and thereby circumvent high voltages. Therefore, large deflections can be obtained in the middle of the membrane.
In order to apply this electrode concept, fabrication of a curved shape becomes the main challenge. In the past, work has been done to fabricate the lateral curved electrode structure using photolithographic techniques. Also, several efforts have been reported to develop a vertical, out-of-plane, curved surface on silicon wafers.
For example, analog lithography and RIE-lag have been used. These past works were successful in creating curved surfaces. However, the fabrication process for these has typically been too complex.
The following articles are related to the above:
R. Legtenberg et al., “Electrostatic Curved Electrode Actuators,” JMEMS, Vol. 6, No. 3, pp. 257–265, 1997;                C. Gimkiewicz et al., “Fabrication of Microprisms for Planar Optical Interconnections by Use of Analog Grayscale Lithography with High-Energy-Beam-Sensitive Glass,” APPLIED OPTICS, Vol. 38, No. 14, pp. 2986–2990, 1999; and        T-K A. Chou et al., “Fabrication of Out-of-Plane Curved Surfaces in Si by Utilizing RIE Lag,” MEMS '02, pp. 145–148, 2002.        
Recently, the usage of polymer materials in MEMS devices has increased considerably because polymers are lighter, more flexible, resistant, cheaper, and easier to process. Polymers such as polyimides, BCB, fluorocarbon polymer, and MYLAR have been used to bond wafers and fabricate 3-D polymer-based microstructures.
Wafer-to-wafer transfer technology has also attracted great attention in applications requiring integration of MEMS with IC, in MEMS packaging cost, and for batch fabrication of 3-D MEMS. In any case, the bonding and detachment of carrier wafer to and from a device wafer are key process technologies. For these purposes, many creative methods have been developed for wafer-level transfer of microstructure from one wafer to another by utilizing wax, SOI wafers, and gold tether bumping.
The following articles are related to the above:
F. Niklaus et al., “Void-Free Full Wafer Adhesive Bonding,” MEMS '01, pp. 214–219, 2001;
A. Han et al., “A Low Temperature Biochemically Compatible Bonding Technique Using Fluoropolymers for Biochemical Microfluidic Systems,” MEMS '00, PP. 414–418, 2000;
Y.-C. Su et al., “Localized Plastic Bonding for Micro Assembly, Packaging and Liquid Encapsulation,” MEMS '01, pp. 50–51, 2001;
E.-H. Yang et al., “A New Wafer-Level Membrane Transfer Technique for MEMS Deformable Mirrors,” MEMS '01, pp. 80–83; and
M. Maharbiz et al., “Batch Micro Packaging by Compression-Bonded Wafer-Wafer Transfer,” MEMS '99, pp. 482–485, 1999.